1. Field of the Invention
Methods consistent with the present invention relate to determining a transmission rate of a control response frame for acknowledging data receipt in a wireless local area network (LAN).
2. Description of the Related Art
In wireless LAN environments, a medium access control (MAC) protocol of a carrier sense multiple access/collision avoidance (CSMA/CA) mechanism is used. The CSMA/CA mechanism is designed to avoid collisions by transmitting a signal when there is no data transmission through a cable of a network and sending data only after it is confirmed that the signal is transmitted without collisions.
The CSMA/CA mechanism works as follows: a terminal attempts to sense a carrier indicating that another terminal is already transmitting data and, if the carrier is sensed, the terminal waits for a random period of time. After the random period of time, the terminal attempts to sense the carrier again. If no other carriers are sensed, the terminal starts to send data.
In the CSMA/CA mechanism, carrier sensing is performed using both physical carrier sensing and virtual carrier sensing. Physical carrier sensing is performed at a physical layer (PHY), which senses whether a received power exceeds a predetermined threshold and informs an MAC layer of whether a medium is “busy” or “idle” based on the sensing result.
In virtual carrier sensing, if an MAC protocol data unit (MPDU) can be accurately extracted from a received PHY packet data unit (PPDU), a “Duration/ID” field, which is one of a plurality of header fields of the MPDU, is interpreted. If the result of an interpretation indicates that a medium is “busy”, the medium is regarded as “busy” for a period of time that the medium is expected to be used. As described above, whether or not a medium is “busy” is determined using the two carrier sensing methods, and the medium is not accessed if busy.
An MPDU/PHY service data unit (PSDU) received must be interpreted normally to effectively apply the virtual carrier sensing method to the CSMA/CA mechanism. In other words, for the virtual carrier sensing method, a value of an MAC header must be read normally. If errors occur due to an unstable channel when data is transmitted at a high transmission data rate, or if a receiving station cannot handle the high transmission data rate, the MPDU/PSDU cannot be interpreted. In this case, virtual carrier sensing is not possible and thus, the CSMA/CA mechanism is ineffective.
FIG. 1 illustrates an IEEE 802.11a-based legacy PPDU frame format. Referring to FIG. 1, if preamble and signal fields in the PPDU frame format are received normally, duration information of a data field can be estimated using rate and length information included in the signal field. Hence, information contained in the preamble and signal fields is useful for a clear channel assessment (CCA) mechanism.
If the preamble and signal fields in the PPDU frame being received are interpreted but a frame check sequence (FCS) error occurs at a receiving station, an MAC layer controls the receiving station to wait for an extended interframe space (EIFS), which is 94 μs in the case of IEEE 802.11a, not a DCF interframe space (DIFS), which is 34 μs in the case of IEEE 802.11a, and backs off.
In other words, if high throughput (HT) stations and legacy stations (802.11a/b/g) with different transmission capabilities coexist on a wireless LAN, the legacy stations cannot interpret an HT frame. Thus, the MAC layers respectively included in the legacy stations fail to accurately carry out virtual carrier sensing and rely only on physical carrier sensing.
Even if the preamble and signal fields of an HT PPDU frame are formatted such that the legacy stations can interpret them, the legacy stations cannot accurately interpret the data field. Thus, the legacy stations have the FCS error and regard the HT PPDU frame as a flawed frame. Then, the MAC layers control the respective legacy stations to wait for the DIFS. On the other hand, stations that can handle a high transmission rate, i.e., the HT stations, can carry out accurate virtual carrier sensing. Thus, the HT stations wait for the DIFS as usual.
Since EIFS=short interframe space (SIFS)+TACK (at the lowest data rate)+DIFS, stations that cannot handle the data rate, i.e., the legacy stations with lower transmission capabilities than the HT stations, are given lower medium access priorities than the HT stations. As a result, medium access fairness for all stations, which is maintained by a distributed coordination function (DCF), cannot be secured.
However, the medium access fairness can be secured when a legacy acknowledgement (ACK) frame is used for acknowledging data transmission on the wireless LAN as illustrated in FIG. 2.
An HT transmitting station HT SRC transmits data to an HT receiving station HT DEST using the HT PPDU frame format. Then, the HT transmitting station HT SRC and the HT receiving station HT DEST wait for a SIFS. After the SIFS, the HT receiving station HT DEST transmits an ACK frame in a legacy format to the HT transmitting station HT SRC to confirm the receipt of data.
When the HT transmitting station transmits the ACK frame in the legacy format, other legacy stations (802.11a-based stations in FIG. 2) as well as the HT receiving station interpret the data field normally. Thus, all stations wait for a DIFS. Consequently, all stations can compete for medium access on an equal footing.
In an IEEE 802.11 standard, if a receiving station supports a frame transmission rate used by a transmitting station, the frame transmission rate is determined to be a transmission rate of a control response frame. If the receiving station does not support the frame transmission rate, the maximum transmission rate among a set of basic transmission rates supported by the wireless LAN is determined to be the transmission rate of the control response frame.
As described above, on the wireless LAN where HT stations and legacy stations with different transmission capabilities coexist, a legacy ACK frame is used for acknowledging data receipt. Accordingly, a transmission rate must be determined using a method different from a conventional method.